Safety devices for electrical apparatus of medical and other types

ABSTRACT

Medical, dental or other apparatus contacting living beings is connected to its source of electrical energy by a safety device which, upon connection to the source, prevents energization of the apparatus if, when the connection is made, faults such as transposed connections, and/or discontinuity in the connections, exist. The safety device includes a differential transformer having a center-tapped primary connected to the hot, neutral, and ground wires of the apparatus, and a normally-closed switch controlling the continuity of the wires. Fault causes the transformer&#39;&#39;s secondary to produce a voltage, which is used to fire an SCR, which opens the switch.

United States Patent 1191 1451 Sept. 25, 1973 SAFETY DEVICES FORELECTRICAL APPARATUS OF MEDICAL AND OTHER TYPES Inventor: Arthur GilbertBillin, Penfield, N.Y.

Assignee: Sybron Corporation, Rochester,

Sept. 10, 1972 Filed:

Appl. No.2

US. Cl. 317/18 D, 317/27 R, 317/31, 317/33 SC, 317/46, 307/127 Int. Cl..1 ,H02h 3/28 Field of Search 317/18 D, 27 R, 31, 317/33 SC, 46, 48, 54;l27/2.06 R; 307/127 References Cited UNITED STATES PATENTS Billin 317/31Primary Examiner-l D. Miller Assistant Examiner-Harvey FendelmanAttorney-Theodore Roessel [57] ABSTRACT voltage, which is used to firean SCR, which opens the switch.

5 Claims, 2 Drawing Figures i 1\ 1O 2O 1 5' SAFETY DEVICES FORELECTRICAL APPARATUS OF MEDICAL AND OTHER TYPES BACKGROUND, FIELD OF THEINVENTION The present invention relates to medical, dental and otherapparatus which may contact a living being unsafely due to fault in theconnection of the apparatus to the electrical source for energizing theapparatus. For example, such connection may be made by polarized plugswhich through wear can be inserted wrongly, so that what is nominallythe hot conductor connecting the hot side of the electrical source isactually the neutral" conductor, and vice versa, that is to say, theconnections are transposed. Again, there may be discontinuity in aground" conductor, for example.

Under various circumstances, these types of connection faults, namely,transposition and discontinuity, may result in leakage currents througha living being contacting the apparatus, and at levels dangerous to thatbeing. In some cases, too, such faults may be harmful to the apparatus.

BACKGROUND, DESCRIPTION OF THE PRIOR ART There are various well-knownexpedients for providing safety in using apparatus of the classdescribed. As noted above, for example, polarized plugs are intended toassure proper connections, and a ground conductor is provided.

In addition, the apparatus may be provided with fault sensing means,such as a ground detector, fuse, or the like, which responds to theexistence of a fault by disconnecting the apparatus; an isolatingtransformer may be interposed between electrical source and apparatus,and so forth.

In particular, US. Pat. No. 3,628,094, granted Dec. 14, 1971 to J..I.Saeli and myself, discloses a safety device for apparatus in whichconnection of the apparatus to the energy source is by means ofa hotconductor and a neutral conductor, and there is also a ground conductor.The patented safety device includes a differential transformer having apair of primary windings, one of which is to be energized by the voltagebetween hot and neutral, the other of which is to be energized by thevoltage I between hot and ground, and each being wound on separatecores. The transformer also has a secondary winding, and this is sorelated to the primary windings and cores that unless there is voltageacross both primaries, and the voltages are properly related a voltageappears across the secondary.

The safety device is preferably more or less integral part of theapparatus with which it is-used. This is because the faults with whichthe invention deals arise ex ternally of the apparatus, and in itsconnections to the electrical source which energizes. Typically, thereis a three-conductor power cord terminating at one end in the apparatus,and at the other end in a plug having male electrodes. A correspondingplug, having female connectors, provides, with the male connectors, theground, neutral, and hot terminals of the system. The safety devicepreferably provides the termination of the power cord at the apparatus,so faults occurring between the apparatus and the energy source willprevent the apparatus from being turned on, if there is a discontinuityor conductor transposition in the connections at any point betweensource and safety device. Thus, either plug may be incorrectly-wired,they may be joined together in wrong polarity, the ground connection maybe broken, and so on, and the safety device will detect this and preventthe apparatus from being manually switched on by the operator of theapparatus.

SUMMARY OF THE INVENTION Like the aforesaid patent, the presentinvention relates to a safety device having a differential transformer.However, the safety device according to the invention includes adifferent form of differential transformer. In particular, the presentinvention provides a single core transformer having a pair of windingsconnected to form a center-tapped primary winding, and

having a secondary winding. The ends of the primary winding areconnected to ground and neutral conductors connecting apparatus to thesource of electrical energy, and the center tap is connected to the hotconductor of the apparatus. Between such connections and the apparatusis a normally-closed switch, which if opened, opens each wire. Theswitch has an operating winding connected between the neutral and hotwires, but via a silicon controlled rectifier (SCR). The secondarywinding of the transformer is connected to the gate of the SCR forfiring same, should a voltage of the correct polarity appear across thesecondary winding. As it happens, no such voltage appears except underfault conditions such as would be dangerous. If such voltage appears,the SCR fires, energizes the operating winding, whereupon the switchopens. As the transformer is a voltage sensing device, appreciablecurrent does not have to flow before the switch opens. The transformerwindings are a single core and very little current need flow inproducing firing voltage for the SCR, so the transformers primary, whichin use is continuously energized, is connected to ground and neutral viacurrent limiting resistance. I

BRIEF DESCRIPTION OF THE DRAWING tral and ground conductors 7, 8 and 9,by means of I which electrically-energizeable apparatus A is to beenergized by a source AC of electrical energy.

Voltage sensing means V is connected at points 7, 8 and 9 to conductors4, 5 and 6, respectively, for controlling a switch S in accordance withthe relationship among the voltages at the said points. Switch S hasmovable contactors 10, 1111 and 12 for conductively bridging theelectrical discontinuity of the respective fixed-contact pairs 13 and114, I5 and 116, and 17 and 18. The dashed-line ganging 19 representsmechanism of switch S for moving contactors ND, 111 and 12.

It is to be supposed that the contactors 10, 1111 and 12 move in unison.It is to be further supposed that points 7, 8 and 9 and the system ofcontacts 13 through 18 are structurally integrated as intimately aspractical with the portions of apparatus A with which they connectelectrically. The first of these portions is represented by impedance Z,representing the electrical load to be driven by source AC, and thesecond of these portions is represented by impedance 2,, representingthat part of the apparatus which is not supposed to be energized.Normally, Z will include the leakage impedance, to housing or chassisstructure, of electrodes, tools, or the like, for contacting a humanbeing.

It will be observed that the only difference between FIG. 1 hereof andFIG. 1 of the aforesaid patent is that in the latter the emphasis was onthe function of the safety device to close contacts of switch 10, if nofaults are sensed. That is still an appropriate consideration in thepresent case, but not a necessary one.

FIG. 2 hereof corresponds to FIG. 2 of the aforesaid patent, and in factdoes not differ therefrom except as to the differential transformer.Thus, in FIG. 2 hereof, windings 30 and 31 are on the same core 34, asis the secondary winding 32. The windings 30 and 32 are connectedtogether so as to define a primary winding having a center-tap, which isconnected to point 7 on hot conductor 4. As indicated by the dots at theadjacent ends of the windings and 31, they are wound in opposition toeach other.

The remaining ends of windings 30 and 31 are connected via resistances37 and 38, respectively to neutral conductor 5 and ground wire 6.

The operating coil or winding 35 of solenoid 130 is connected across hotconductor 4 and neutral conductor 5 via an SCR 36. Secondary winding 32is connected from the gate of the cathode of the SCR. Consequently,unless there is induced a sufficiently large and properly polarizedvoltage across winding 32, the SCR will prevent current flow throughcoil 35. When SCR 36 conducts, diode 39, across coil 35, provides a pathfor reverse current when the SCR is in the nonconductive half of thepower cycle and provides quiet operation of the relay.

The overall functioning of FIG. 2 hereof is generally as in FIG. 2 ofthe aforesaid patent, so it will not be necessary to describe theremainder of the Figure here. The basic difference between the two isthat the solenoid 130 drew its power from the line via the transformer,in the earlier case, whereas in the present case, the solenoid draws itspower directly from the line, under direct control of the SCR 36.Consequently, in the present case, the safety device can useaconventional single core transformer, rather than the rather special(and more bulky) two-core transformer of the patent because the currentlimiting resistors prevent excessive currents from flowing in theopposing primaries.

The transformer of the patent also had to draw continuously a relativelylarge current from the line, whereas in the present case, the currentlimiting resistances 37 and 38, typically 50,000 ohms each, or more,limit the current drawn from the transformer. Only a few hundredmicroamperes of current at a low voltage suffice to fire the SCR 36,whereas the transformer of the patent has to be able to supply severalhundred times as much energy to energize the solenoid 130 properly.

While the SCR is an added element to be taken accound of in consideringthe reliability of the safety device, the normal failure habit ofan SCRis to short its anode/cathode junction, so it is substantially failsafe, because such shorting will open the contacts 10, 11 and 12.

In any event, as long as conductors 5, 6 and 7 are properly connectedand energized, the voltage across winding 30 will cancel the voltageacross winding 31, and hence there will be substantially zero voltageacross winding 32, and therefore SCR 36 will not fire. On the otherhand, the voltages across windings 30 and 31 will differ enough that SCR36 fires when terminal 1 goes positive enough with respect to terminal12, under the following conditions:

1. Electrical discontinuity in conductor 5.

2. Electrical discontinuity in conductor 6.

3. Interchanged/connections of conductors 4 and 6 to terminals 1 and 2.

4. Interchanged/connections of conductors 4 and 5 to terminals 1 and 2.5. Interchanged/connections of conductors 5 and 6 to terminals 2 and 3,if the latter are at different potentials.

The SCR can be replaced by a Triac or other gatecontrolled solid statedevice capable of controlling relatively high current, in response tolow gate voltage.

Having described my invention according to the requirements of 35 USC112, I claim:

1. The combination with an electrically-energizeable apparatus adaptednormally to electrically contact a living being safely when saidapparatus is electricallyenergized and free of fault, of a safety deviceoperatively associated with said apparatus for preventing fault fromcausing said apparatus to be in unsafe electrical contact with saidliving being;

said apparatus having a hot conductor, a neutral conductor and a groundconductor, said hot conductor being connected to a first portion of saidapparatus and being adapted to connect said first portion to the hotterminal of a source of electrical energy, said neutral conductor beingconnected to said first portion and being adapted to connect said firstportion to the neutral terminal of said source, said ground conductorbeing adapted to connect a second portion of said apparatus to a groundterminal;

there being means, independent of said apparatus, electricallyinterconnecting said neutral terminal and said ground terminal, and saidapparatus being energizeable in response to the electrical energy ofsaid source available at said terminals;

said safety device including a switch controlling electrical continuityof said hot conductor at a place between the said hot terminal and saidfirst portion; said safety device also including voltage sensing meansconnected in voltage sensing relation to said conductors at pointsbetween said portions, on the one hand, and said terminals, on the otherhand; said voltage sensing means being connected to said switch andbeing responsive to the several voltages at said points to substantiallyprevent said switch from being operative to control electricalcontinuity at said place, under any one of the following conditions:electrical discontinuity in said neutral conductor; electricaldiscontinuity in said ground conductor; connecting said hot conductor tosaid ground terminal and said ground conductor to said hot ter minal;connecting said hot conductor to said neutral terminal and said neutralconductor to said hot terminal; connecting said neutral conductor tosaid ground terminal and said ground conductor to said neutral terminal,provided said neutral and ground terminals are at different potentials;said voltage sensing means including a core, primary windings on saidcore, a secondary winding on said core, and first and second currentlimiting means; one said primary winding and said first current limitingmeans being connected in series between said hot conductor and saidneutral conductor for producing first flux in said core when there isvoltage between said hot conductor and said neutral conductor;

a second said primary winding and said second current limiting meansbeing connected in series between said hot conductor and said groundconductor for producing second flux in said core when there is voltagebetween said hot conductor and said ground conductor;

said one said primary winding and said second said primary winding beingarranged on said core so that the fluxes produced thereby cancel whenthe voltages on said conductors have the correct relative polarities andmagnitudes;

said switch having operating means electrically interconnecting said hotconductor and said neutral conductor, said operating means including anSCR interrupting such electrical interconnection except when said SCR isfiring, and there being means electrically connecting the gate of saidSCR to said secondary winding, for firing said SCR in response tovoltage of the correct polarity and magnitude appearing across saidsecondary winding.

2. The invention of claim 1, wherein said first current limiting meansincludes a first resistor having one end connected to said neutralconductor and having its other end connected to said one said primarywinding, and said second current limiting means includes a secondresistor having one end connected to said ground conductor and its otherend connected to said second said primary winding.

3. Switch operating circuitry including, in combination:

a transformer having a core, a primary winding having a center tap, anda secondary winding, said windings being wound on said core, and saidprimary winding being a first winding and a second winding, each havingone end connected to the other and being wound in a sense opposite tothat of the other;

an SCR, said SCR having its anode connected to said center tap, its gateconnected to one end of said secondary winding, and its cathodeconnected to the other end of said first winding and to the other end ofsaid secondary winding; and

an operating coil for a switch, said coil being connected in series withthe said anode and cathode of said SCR.

4. The switch operating circuitry of claim 3, wherein there is firstcurrent limiting means interconnecting said other end of said firstwinding'to said cathode and to said other end of said secondary winding,and there is second current limiting means connected to the other end ofsaid second winding.

5. The switch operating circuitry of claim 4, wherein said first currentlimiting means is a first resistor, and

said second current limiting means is a second resistor. 4: =0: =0:

1. The combination with an electrically-energizeable apparatus adaptednormally to electrically contact a living being safely when saidapparatus is electrically-energized and free of fault, of a safetydevice operatively associated with said apparatus for preventing faultfrom causing said apparatus to be in unsafe electrical contact with saidliving being; said apparatus having a hot conductor, a neutral conductorand a ground conductor, said hot conductor being connected to a firstportion of said apparatus and being adapted to connect said firstportion to the hot terminal of a source of electrical energy, saidneutral conductor being connected to said first portion and beingadapted to connect said first portion to the neutral terminal of saidsource, said ground conductor being adapted to connect a second portionof said apparatus to a ground terminal; there being means, independentof said apparatus, electrically interconnecting said neutral terminaland said ground terminal, and said apparatus being energizeable inresponse to the electrical energy of said source available at saidterminals; said safety device including a switch controlling electricalcontinuity of said hot conductor at a place between the said hotterminal and said first portion; said safety device also includingvoltage sensing means connected in voltage sensing relation to saidconductors at points between said portions, on the one hand, and saidterminals, on the other hand; said voltage sensing means being connectedto said switch and being responsive to the several voltages at saidpoints to substantially prevent said switch from being operative tocontrol electrical continuity at said place, under any one of thefollowing conditions: electrical discontinuity in said neutralconductor; electrical discontinuity in said ground conductor; connectingsaid hot conductor to said ground terminal and said ground conductor tosaid hot terminal; connecting said hot conductor to said neutralterminal and said neutral conductor to said hot terminal; connectingsaid neutral conductor to said ground terminal and said ground conductorto said neutral terminal, provided said neutral and ground terminals areat different potentials; said voltage sensing means including a core,primary windings on said core, a secondary winding on said core, andfirst and second current limiting means; one said primary winding andsaid first current limiting means being connected in series between saidhot conductor and said neutral conductor for producing first flux insaid core when there is voltage between said hot conductor and saidneutral conductor; a second said primary winding and said second currentlimiting means being connected in series between said hot conductor andsaid ground conductor for producing second flux in said core when thereis voltage between said hot conductor and said ground conductor; saidone said primary winding and said second said primary winding beingarranged on said core so that the fluxes produced thereby cancel whenthe voltages on said conductors have the correct relative polarities andmagnitudes; said switch having operating means electricallyinterconnecting said hot conductor and said neutral conductor, saidoperating means including an SCR interrupting such electricalinterconnection except when said SCR is firing, and there being meanselectrically connecting the gate of said SCR to said secondary winding,for firing said SCR in response to voltage of the correct polarity andmagnitude appearing across said secondary winding.
 2. The invention Ofclaim 1, wherein said first current limiting means includes a firstresistor having one end connected to said neutral conductor and havingits other end connected to said one said primary winding, and saidsecond current limiting means includes a second resistor having one endconnected to said ground conductor and its other end connected to saidsecond said primary winding.
 3. Switch operating circuitry including, incombination: a transformer having a core, a primary winding having acenter tap, and a secondary winding, said windings being wound on saidcore, and said primary winding being a first winding and a secondwinding, each having one end connected to the other and being wound in asense opposite to that of the other; an SCR, said SCR having its anodeconnected to said center tap, its gate connected to one end of saidsecondary winding, and its cathode connected to the other end of saidfirst winding and to the other end of said secondary winding; and anoperating coil for a switch, said coil being connected in series withthe said anode and cathode of said SCR.
 4. The switch operatingcircuitry of claim 3, wherein there is first current limiting meansinterconnecting said other end of said first winding to said cathode andto said other end of said secondary winding, and there is second currentlimiting means connected to the other end of said second winding.
 5. Theswitch operating circuitry of claim 4, wherein said first currentlimiting means is a first resistor, and said second current limitingmeans is a second resistor.